Terminology clarification: "Wireless" is how Kindle connects to the world. Amazon call it "Whispernet". It can be via WiFi or via 3G. Either way, you're connecting wirelessly.

If you use WiFi, it doesn't cost Amazon anything.
If you use 3G, it does. That's part of the reason the Kindle with both receivers costs more money.
I use WiFi at home and anywhere else that I can connect that way. But I do like having 3G just in case or if I don't want to go to the trouble to work through a WiFi connection someplace where I'm only going to use it once. But for my regular haunts, I set it up once and then it works every time I'm at one of those places.